The use of palladium and palladium-rich alloys for the separation of hydrogen from hydrogen-containing streams is well known. Equipment based on the permeation of hydrogen through pure palladium and later on through palladium-rich alloys has been sold commercially for at least twenty-five years and patents in this art go back to 1919.
It is a necessary characteristic of palladium and of all hydrogen permeation alloys that hydrogen dissolves into the bulk material. It is a characteristic of all practical permeation materials to date that the amount of hydrogen which dissolves into the material increases as the temperature decreases. It is another common characteristic that, as the amount of hydrogen in the metal increases, so does the spacing between metal atoms and so does the size of the bulk material. For practical reasons, permeation materials are used in thin section. Permeation rates are inversely proportional to barrier thickness and weights and associated precious metal costs are directly proportional to thickness. The permeation septa, generally in the form of tubing, are joined to massive structures which do not dissolve hydrogen in appreciable quantity, and which do not expand at low temperatures in the presence of hydrogen. For this reason, all current commercial hydrogen permeation equipment is subject to a destructive mismatch of expansion at the palladium alloy joint on cooling in the presence of hydrogen. Such destructive conditions can occur during long term power failure or because of faulty operating procedures.
It is a further characteristic of permeation barrier materials that hydrogen moves from a high hydrogen pressure on one side of the barrier to a lower hydrogen pressure on the other side of the barrier. It makes no difference to the permeation rate whether the far side of the barrier is under vacuum or has a high total pressure of non-permeable gas. As an example, hydrogen at one atmosphere will diffuse through a barrier into a volume which contains several psi or several thousand psi of nitrogen or argon. It will stop permeating through the barrier only when hydrogen pressures are equalized on both sides of the barrier.